A solid light source lighting device as a prior art is exemplified in JP Pub. No. 2012-221899 (hereinafter referred to as “Document 1”). The solid light source lighting device described in Document 1 includes a buck chopper circuit, and is configured to dim, namely adjust a light output of a solid light source (e.g., an LED (Light-Emitting Diode) light source) by controlling the buck chopper circuit so that an output current to the solid light source is increased and decreased.
With the solid light source lighting device described in Document 1, a load current (an electric current flowing through the solid light source) is increased and decreased by lengthening and shortening (increasing and decreasing) an ON time (an ON duty ratio) of a switching device (a semiconductor device such as a transistor) forming the buck chopper circuit. A dimming method as stated above is called a DC dimming method in general. With the DC dimming method, there is a limit (a lower limit) in an ON period (an ON width) of a drive signal supplied from a drive circuit for driving the switching device to a control terminal (a gate terminal) of the switching device, thereby making it difficult to perform deep dimming.
As another dimming method, there is also a dimming method with a dropper regulator (hereinafter referred to as a “linear dimming method”). In comparison with a switching regulator, the dropper regulator has an advantage of low ripple and low noise, but has disadvantages of low conversion efficiency and the like. With the linear dimming method, a load current is increased and decreased by changing ON resistance of a field effect transistor, for example. The linear dimming method accordingly has an advantage of making it possible to perform deeper dimming than dimming by the DC dimming method.
Therefore, in a solid light source lighting device (an LED driver) that employs the DC dimming method, the DC dimming method is switched to the linear dimming method in order to perform deep dimming.
However, in the case of the linear dimming method, loss increases according to an increase in a difference between input and output voltages of the LED driver, thereby decreasing the conversion efficiency.